Functional and Genetic Diversity of Bacteria Associated with the Surfaces of Agronomic Plants

plants Article Functional and Genetic Diversity of Bacteria Associated with the Surfaces of Agronomic Plants Basharat Ali Department of Microbiology and Molecular Genetics, University of the Punjab, Lahore 54590, Pakistan; [email protected]; Tel.: +92-300-4297937 Received: 9 February 2019; Accepted: 2 April 2019; Published: 4 April 2019 Abstract: The main objective of this study was to evaluate the genetic diversity and agricultural significance of bacterial communities associated with the surfaces of selected agronomic plants (carrot, cabbage and turnip). The bacterial diversity of fresh agricultural produce was targeted to identify beneficial plant microflora or opportunistic human pathogens that may be associated with the surfaces of plants. Bacterial strains were screened in vitro for auxin production, biofilm formation and antibiotic resistance. 16S rRNA gene sequencing confirmed the presence of several bacterial genera including Citrobacter, Pseudomonas, Pantoea, Bacillus, Kluyvera, Lysinibacillus, Acinetobacter, Enterobacter, Serratia, Staphylococcus, Burkholderia, Exiguobacterium, Stenotrophomonas, Arthrobacter and Klebsiella. To address the biosafety issue, the antibiotic susceptibility pattern of strains was determined against different antibiotics. The majority of the strains were resistant to amoxicillin (25 µg) and nalidixic acid (30 µg). Strains were also screened for plant growth-promoting attributes to evaluate their positive interaction with colonized plants. Maximum auxin production was observed with Stenotrophomonas maltophilia MCt-1 (101 µg mL−1) and Bacillus cereus PCt-1 (97 µg mL−1). Arthrobacter nicotianae Lb-41 and Exiguobacterium mexicanum MCb-4 were strong biofilm producers. In conclusion, surfaces of raw vegetables were inhabited by different bacterial genera. Potential human pathogens such as Bacillus cereus, Bacillus anthracis, Enterobacter cloacae, Enterobacter amnigenus and Klebsiella pneumoniae were also isolated, which makes the biosafety of these vegetable a great concern for the local community. Nevertheless, these microbes also harbor beneficial plant growth-promoting traits that indicated their positive interaction with their host plants. In particular, bacterial auxin production may facilitate the growth of agronomic plants under natural conditions. Moreover, biofilm formation may help bacteria to colonize plant surfaces to show positive interactions with host plants. Keywords: food biosafety; bacterial colonization; antibiotic resistance; biofilm formation; raw-eaten vegetables; bacterial auxin production 1. Introduction Fresh vegetables and fruits are integral components of the human diet and consumed in sufficient amounts to maintain good health [1]. There has been a correlation between foodborne outbreaks and increased production, imports and the consumption of fresh agricultural produce [2]. Raw vegetables can harbor pathogenic microorganisms that can cause serious illnesses to humans after consumption. Routes of microbial contamination include the application of organic wastes to soil as fertilizers, use of fecal-contaminated water for irrigation, direct contact with livestock and post-harvest hygiene issues [3]. In most common practices, fresh fruits and vegetables are consumed raw or processed to minimize the chances of foodborne outbreaks. The frequency of foodborne outbreaks from the consumption of contaminated vegetables and fruits has increased in recent decades. There is a long list of reports that confirms that raw vegetables foster several potentially pathogenic organisms, indicating their key involvement in fresh produce-associated outbreaks [4–9]. Plants 2019, 8, 91; doi:10.3390/plants8040091 www.mdpi.com/journal/plants Plants 2019, 8, 91 2 of 15 True or opportunistic human pathogens have been found and reported with foodborne outbreaks due to their adaptation to soil and plant surfaces [10]. They may be highly competitive for nutrients and produce antimicrobial agents to suppress the native microflora to colonize plant surfaces. For instance, pathogenic strains of Pseudomonas aeruginosa and Stenotrophomonas maltophila are well documented to colonize wheat and strawberries, respectively [11]. A similar ability of Burkholderia cepacia to cause infections in plants and humans has also been reported [12]. Besides the true human pathogens (Escherichia coli and Salmonella enterica), a number of potential human pathogens such as Achromobacter xylosoxidans, Janthinobacterium lividum, Alcaligenes xylosoxidans, Alcaligenes faecalis, Serratia marcescens, Staphylococcus aureus, Enterobacter amnigenus, Bacillus cereus, and Stenotrophomonas maltophilia have been reported to be present in plant-associated environments [13,14]. Plant-associated microorganisms can conveniently be categorized in three groups: plant-beneficial, plant pathogenic and opportunistic human pathogenic organisms. The plant-beneficial group of bacteria comprises members of different genera such as Burkholderia, Enterobacter, Stenotrophomonas, Pseudomonas, Herbaspirillum, Ralstonia, Bradyrhizobium, Ochrobactrum, Bacillus, Rhizobium and Staphylococcus. These microorganisms are mostly root-associated and factor in the bivalent interactions with plants and human or animal hosts [15]. The microorganisms of these genera proved themselves beneficial for plants by playing their role in growth promotion, protection from phytopathogens and provision of different elemental nutrients such as nitrogen and phosphorus [16,17]. Many potential pathogens of humans live in close association with plants and, therefore, have adapted to benefit plants in several ways in order to ensure their persistence. They can produce different vital phytohormones such as auxin, gibberellins and cytokinins. Some potentially pathogenic strains of Bacillus and Serratia are noted in the growth promotion of diverse crops such as potato, wheat and maize [18,19]. Therefore, the main objective of the present study was to evaluate the microbiological biosafety and agronomic significance of bacterial species associated with raw vegetables. For this purpose, enriched and selective culture media were used to isolate general and specific bacterial species associated with fresh produce. The final taxonomic status of the bacterial isolates was confirmed by 16S rRNA gene sequencing [20]. Fresh produce-associated microbial hazards have not been uncovered in detail in Pakistan. Therefore, in this study, bacterial diversity associated with carrot, cabbage and turnip was targeted. The main reason to work with these agronomic plants was their poor handling from field to local markets. This may have resulted in their cross contamination with potential human pathogens during transportation. It was also hypothesized that after interacting with host plants, bacterial species may have developed beneficial plant growth-promoting traits. With this in mind, the phylogenetic diversity and agricultural significance of bacterial communities were also inspected to identify the range of bacterial species showing positive interactions with colonized plants. 2. Materials and Methods 2.1. Sample Collection and Culture Media Samples of carrot, cabbage and turnip (12 samples each) were collected from different sites in Lahore, Pakistan. Samples were collected from 12 randomly selected sites that were comprised of small vegetable shops, major vegetable markets and superstores. Samples were collected in sterile plastic bags and brought to the laboratory and processed within 24 h. In order to assess the bacterial diversity, different types of culture media including Luria–Bertani Agar (L-agar), Baird–Parker Agar (BPA), Eosin Methylene Blue Agar (EMB), Mannitol Salt Agar (MSA), MacConkey Agar (MAC) and Xylose Lysine Deoxycholate Agar (XLD) were used. L-agar is extensively used as a general and non-selective growth medium [21]. BPA and MSA are selective for the growth of Gram-positive bacteria specifically for Staphylococcus aureus. EMB, MAC and XLD support the growth of Gram-negative bacteria. EMB agar, in particular, differentiates between lactose and non-lactose fermenter groups of Gram-negative bacteria and is widely used for the isolation of E. coli, Citrobacter freunddii and Enterobacter cloacae. Plants 2019, 8, 91 3 of 15 XLD agar is particularly used for the isolation of Salmonella enterica and Shigella flexneri from food and clinical samples [22]. 2.2. Isolation of Bacterial Strains Samples were crushed in sterile mortars and pestles and diluted serially before spreading. For serial dilution, 10 g of the crushed sample was added into pre-autoclaved 100 mL normal saline solution (0.85% NaCl) and mixed well. Approximately 50 µL of this dilution was aseptically plated on the surface of L-agar and other selective media as mentioned above. After spreading, the plates were incubated between 30 to 37 ◦C for 24 h. Following incubation, colonies with different morphologies were selected and purified using several rounds of streaking on respective media (Figure S1). The purity of the colonies was also monitored by performing Gram staining. 2.3. 16S rRNA Gene Sequencing For 16S rRNA gene sequencing, genomic DNA was extracted from bacterial cultures grown overnight using a Genomic DNA Purification Kit (Promega, USA). An approximately 1·5-kb DNA fragment containing 16S rRNA gene was amplified using forward 27f and reverse primer 1522r [23]. PCR amplification was performed using 50 µL of Dream TaqTM Green PCR Master Mix (ThermoFisher Scientific, Chelmsford, MA, USA) with 0.5 µg of chromosomal DNA template

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